LACUSTRINE ORGANIC GEOCHEMISTRY - AN OVERVIEW OF INDICATORS OF ORGANIC-MATTER SOURCES AND DIAGENESIS IN LAKE-SEDIMENTS

The factors affecting the amounts and types of organic matter in lacustrine sediments arc summarized in this review, and synthesis, of published studies. Biota living in the lake and in its watershed are the sources of the organic compounds initially contributed to the lake system. Microbial reworking of these materials during sinking and early sedimentation markedly diminishes thd total amount of organic matter while replacing many of the primary compounds with secondary ones. Much of the organic matter content of sediments is the product of this Microbial reprocessing. Various organic matter components of lake sediments nonetheless retain source information and thereby contribute to thd paleolimnological record. Carbon/nitrogen ratios of total organic matter reflect original proportions of algal and land-derived material. Carbon isotopic compositions indicate the history of lake productivity and carbon recycling. Biomarker compounds provide important information about contributions from different biota. Sterol compositions and chainlength distributions of n-alkanes, n-alkanoic acids, and n-alkanols help distinguish different algal and watershed sources and also record diagenetic alterations. Stabilization of functional-group-containing biomarkers by conversion into saturated or aromatic hydrocarbons or by incorporation into bound forms improves their preservation and hence record of source information. Lignin components provide important evidence of watershed plant cover, and pigments reflect algal assemblages. The interplay of the factors influencing thd organic matter content of lake sediments is illustrated by overviews of sedimentary records of four lake systems-Lake Biwa (Japan), Lake Greifen (Switzerland), Lake Washington (Pacific Northwest), and the Great Lakes (American Midwest).